[4.0] Request: Change of Columns handling in Backend Forms

[marcorensch]

based on: #29706

So fresh wind here: What if:

• The columns will been used when a dev groups its fields additionaly by fieldsets:
  < fieldset name="myTab">
  < fieldset name="basics">
  < field>< /field>
  < /fieldset>
  < fieldset name="borders">
  < field>< /field>
  < /fieldset>
  < /fieldset>

And when no fieldsets are used INSIDE the main fieldset the columns are limited to one column...
every additional fieldset (level 3 and after) should be ignored by this changes

This solution could have this benefits:

• the dev can define if she / he likes to use columns
• columns count would / could still be possible to define if another PR will be created to define the columns
• both sides got what they want to have (multiple columns and single column)
• "Jumping issue" for hidden and later visible fields are not solved in general, yes but it will not happen anymore with this structure
• you have nothing to change for existing J3 extensions XML to migrate

In my test it was necessary to put div's arround the fieldsets to have a nice looking GUI

In my test the generated HTML would need to look like this:
https://ibb.co/PWhx0V7
https://ibb.co/6Yw0Pjx

Things ToDo for this:

• Check in Form Renderer for fieldsets in main fieldsets
  -- Add the column classes as already been done in the actual release & wrap fieldset element inside a div
  -- Otherwise add column class for 1 column (because there are only fields and all of you are happy)
• Small CSS changes (add rule to remove display inline block for sub sub fieldsets
  -- gets actually broken by [class^="column-"] > div, [class*=" column-"] > div {
  /* display: inline-flex; ... } in template.css on 10062 (Beta 5) (disable in console while live testing and it worked like a charm)

And yes a small note inside the J4 docs would also be necessary to understand the logic and as i can see it should be backwards compatible too.

I'm sorry but i have no clue how to create build that for you to testing - but the GUI changes can be simulated by changing the HTML directly in the browser. What do you guys think?

Originally posted by @marcorensch in #29706 (comment)